This invention relates generally to satellite navigation systems, and more particularly to controlling solution latency in a satellite navigation system.
Global Navigation Satellite Systems (GNSS) are well known and used to solve a wide variety of positioning/time related tasks. Two well known such systems are the Global Positioning System (GPS) of the United States and the GLObal NAvigation Satellite System (GLONASS) of Russia. This description will generally refer to the GPS system, but it is to be understood that the present description is equally applicable to GLONASS or other GNSS systems.
In recent years, there have been significant developments in the use of GPS receivers in the surveying and construction industries. In surveying applications, GPS is generally used to determine locations of points on the ground. GPS receivers are also being used for machine control in which the GPS position information is used to control construction machines. For example, GPS positioning information may be used to dynamically control a bulldozer blade. The use of GPS positioning in surveying and machine control provides many benefits, including responsiveness, reliability, autonomy, and all-weather operation.
The accuracy of GPS positioning has been improved by integrating other sources of positioning information with the GPS positioning receiver. These other sources provide additional positioning information which allows for a more accurate positioning solution. One example of such integration is in aviation, where it is common to integrate inertial sensors with GPS receivers. One reason for such integration is that inertial sensors have small short-term error and large long-term error, while GPS receivers have large short-term error and small long-term error. The integration of inertial sensors with GPS receivers provides a complement and allows a combined device to reduce both short-term and long-term errors. This improves the accuracy of positioning determination.
Typical examples of inertial sensors are accelerometers and gyros. Accelerometers measure acceleration and gyros measure angular rate. One typical integration technique is to provide an Inertial Measurement Unit (IMU) consisting of three accelerometers and three gyros. Sensors in each triad are orientated in mutually perpendicular directions. The sensor signals are digitized using an Analog to Digital Converter (ADC) and provided to the GPS receiver where they are combined with the GPS data.
An important requirement of an integrated IMU/GPS system is time synchronization of the GPS and IMU data. In other words, the data that is generated by each of the systems should have the same time reference point. Otherwise, the benefits of integrating the two systems will be reduced because there will be a reduction in accuracy of the ultimate positioning result obtained. In an IMU of the type described above, synchronization is generally accomplished as follows. The GPS receiver calculates position information at periodic time moments, referred to herein as epochs, which are defined by a local clock signal within the GPS receiver. The local clock signal is formed using a high quality quartz generator. The GPS receiver outputs this local clock signal to the IMU. The IMU outputs its data to the receiver in response to the local clock signal. The IMU and GPS are synchronized by controlling the timing of the IMU's data generation by the GPS clock signal.
There are sources of positioning and geometric information which are not so easily synchronized. For example, there exist devices which provide positioning/geometric information where the precise moment of data generation cannot be controlled via a GPS clock signal. One such device is a rotating laser system which may be used to determine additional positioning/geometric information. Such a system is described in detail in co-pending U.S. patent application Ser. No. 10/732,145 filed Dec. 10, 2003, entitled Working Position Measuring System. Such a rotating laser system generally includes a rotating laser at a fixed location, with a photodetector co-located with the GPS receiver. The photodetector periodically detects the rotating laser beam and generates a signal based upon receipt of the laser (i.e., when the laser beam strikes a photocell of the detector). The signal may be processed using various techniques in order to provide additional positioning/geometric information, such as the elevation angle between the photodetector and the transmitter. Knowledge of the elevation angle allows calculation of the height difference between transmitter and photodetector by multiplying the tangent of the angle and distance. Distance is calculated by the GPS receiver which is integrated in one housing with the laser detector. The accuracy of height estimation is improved because the elevation angle measurement is very precise within a small range. Such techniques are described fully in the above reference co-pending U.S. patent application Ser. No. 10/732,145. The details of such techniques are not important for the present discussion. What is important to recognize is that the data generation of the photodetector may not be synchronized with the GPS system by providing the GPS clock signal to the photodetector. The timing of the data generation of the photodetector is dependent upon the moment in time that the laser beam strikes the photocell of the photodetector. Such time is dependent upon the angular velocity of the rotating laser transmitter (and dependent upon dynamics (e.g., movement, if any) of the photodetector) and cannot be controlled by a clock signal received by the photodetector.
The above described synchronization problem results in a degradation of the position calculation. At the epoch moment when the GPS position calculation is made, the elevation angle data available from the photodetector is from some point in time prior to the epoch moment. The elevation angle is likely to have changed since this prior point in time, and thus the use of the old elevation angle data inserts error into the position calculation. Thus, stated another way, there is a latency between the time the elevation angle is calculated and the time when such elevation angle is actually used in conjunction with the GPS data to determine position. This solution latency results in reduced positioning accuracy.
One known solution to this problem is to extrapolate the elevation angle data to estimate the elevation angle at the epoch moment when the positioning determination is made. While this is an improvement over using the earlier calculated elevation angle, there is still a significant extrapolation error which reduces the accuracy of the position determination. In dynamic machine control applications, such as bulldozer blade control, such reduced accuracy has a significant effect on the overall effectiveness of the system.
While the solution latency problem has been described in connection with a rotating laser system which computes elevation angle, one of ordinary skill in the art will recognize that the latency problem exists anytime a non-satellite source of positioning information is integrated with a GPS system where the non-satellite source cannot be readily synchronized using a clock signal from the GPS receiver.
Thus, there exists a need to solve the solution latency problem described above.